This invention refers to a process and a device for confining, retaining and drawing off vapors, fumes, dust or similar fluid materials, resulting from kitchen ovens, cooking places and industrial working stations. However, the invention also can be used for retaining and drawing off other fluid materials, such as solutions, dispersions or suspensions. Especially, the invention refers to exhaust hoods to be used in the kitchen field and in the field of clean-air rooms.
Vapors, dust, fumes and the like are pollutants to be removed from air by drawing off the pollutants through a filter, for example a vapor hood. These materials often are included in very fast and turbulent streams of air. A mere suction flow is not suitable for retaining such streams because it""s intensity nor it""s stability is sufficient to divert and draw off a turbulent flow. For this reason the volume of the drawing off stream is chosen to be considerably larger than the volume stream of the pollutants, or a larger suction screen with high suction power is used.
DE 39 18 870 C2 discloses a method for improving the suction flow field of an exhaust hood. A downwardly directed free jet and a wall jet directed towards the suction surface cooperate with each other and generate a frontal vortex. The flow field produces an aerodynamic wall around the exhaust hood.
DE 42 03 916 C1 discloses a process for forming a blast flow according to DE 39 18 870 in such a manner that it results in a higher inherent stability and is formed as a helix, and passes the frontal vortex along the lateral sides of the exhaust hood. The drawback of both described processes is the expensive structure of a twin slot nozzle for generating the frontal vortex and the wall jet, as well as the problem in diverting the frontal vortex at the edges of exhaust hoods.
DE 33 04 262 C2 discloses a recirculation hood forming an air curtain around the lateral walls of an exhaust hood. As can be seen from the Schlieren photographs of this type of air curtain, no distinguished front is generated. Especially, this type of recirculation hood is to be further developed and improved by the subject invention.
In the field of meteorology, the expression xe2x80x98frontxe2x80x99 is the boundary between different air masses. A front is a strongly convergent flow area on which extreme gradients, for example temperature or moisture gradients, preferably adjacent to boundary surfaces, such as the ground or a wall can occur. According to the invention, this type of front also is generated as an area of flow between the vapor section and the blow out area of the exhaust hood.
It is an object of this invention to improve the suction flow field of an exhaust hood for vapors, fumes and dust in such a manner that vapors, fumes, and/or dust are separated from ambient air, and a front is formed.
A blast jet exiting around the front edge of the hood is diverted into a movement extending towards the suction surface, and is transformed into a vortex or a curved shearing flow or shearing layer. Ideally, a vortex comprises a fixedly rotating nucleus surrounded by a shearing flow or a shearing layer. For generating a front it is vital that the shearing flow is able to build a convergent stream field generating a front if the stream hits a wall or a counterstream. With the invention front vortexes, as well as vortex or shearing streams are generated, and devices are proposed which build a front on the underside of the exhaust hood in a more stable and more effective manner, and also generate helical suction flows.
Diverting a jet for obtaining a curved vortex or shearing flow is obtained in different ways:
1. A direct suction effect acts upon a jet. The jet is blown out at the front edge of the hood into the area below the hood and, by means of gap suction formed within the deeper inner edge area of the hood, is diverted towards the bottom surface of the hood. Optimum orientation of the jet depends upon the intensity and the distance of the edge suctioning from the blast slot. Preferably, the jet is orientated in an angle of +/xe2x88x9230xc2x0 to the vertical in order to obtain a positive generation of a front vortex and a front layer. The aperture of the suction slot is provided towards the center of the hood. The exit aperture and suction aperture in their most simple embodiments are separated from each other by a straight surface, the distance depending on the radius of curvature. The suction rate is in the order of the exhaust rate and is approximately 3-5 m/sec. In front of the suction slot, a trough can be arranged to act as a receiving trough and as a means for diverting the suctioned free jet and the vapor elements pulled along with the free jet.
2. The jet diversion is caused by the action of the Coanda effect on a wall jet over a curved surface or by blowing the jet in an inclined direction over a plane surface. The suction effect causing a curvature and acting upon a free jet also can be generated by a free jet by blowing the jet over a curved surface. Such jet adheres to the curved surface and is diverted up to 240xc2x0. This effect is known as the so-called Coanda effect and generates a vortex flow or a shearing flow. The curved surface takes the function of a vortex nucleus, either partly or totally. If a break edge is provided within the curvature, a vortex can be generated at this break edge. A jet is directed outwardly over a circular profile or a partial circular profile in a horizontal direction and generates a flow which at the bottom side of the hood is directed against the interior of the hood. For improving the adherence of a wall jet on a curved surface boundary layer, suctioning can be provided within the area where the flow separates from the surface.
A further possibility to provide a jet with a curved direction is to blow the jet at an angle a in view of the exit direction towards an inclined plate, a correspondingly inclined profile or a curvature if the jet adheres to the plate at an angle of 0 less than xcex1 less than 50xc2x0. This is possible with a plate which is approached with the indicated range of angles. The jet adheres in a distance of 5-30% of the thickness of the blow out slot behind the slot at an angle of 25xc2x0 less than xcex1 less than 30xc2x0.
Another way of diverting the jet is to blow it onto a straight surface in a tangential direction, which means that xcex1=0xc2x0, and the jet is a wall jet. This plane surface is joined by a curvature or a profile in order to generate a corresponding flow. If a half-circular, a circular segment type, a profiled or a similarly curved element is inserted between the vertical blast jet and the horizontal wall jet of a nozzle according to DE 39 18 870 C2, the effect of the process according to the invention will improve because the nucleus of the generated front vortex does not need to be built up completely or partly. Therefore, a larger proportion of the jet can be transformed into a vortex flow for generating a front.
3. Another possibility of diverting the jet is to direct a free jet leaving the front edge of the hood against a profile in such a manner that the jet is diverted in the direction towards the bottom side of the hood and towards the interior of the hood so that a curved vortex or shearing flow is generated. Such diversion of the jet towards the bottom side of the hood is equivalent to the effect of a flat of an airplane which, at high angles of incidence, passes the approach flow towards the airfoil profile.
4. A fourth possibility of diverting the jet is to combine the generation of a front vortex or a front vortex type flow according to the above possibilities mentioned in items 2 and 3 with edge suctioning according to item 1, whereby surface suctioning can be dispensed with.
If several fans are used within an exhaust hood, either all fans can be operated in the suction mode, and part of the suction air can be diverted, or alternatively, separated fans for suctioning off or fans for blowing out vapors are used. The former process lends itself to an option if a single fan is used. When operating in the air outlet mode there is the disadvantage that the amount of air for the jet depends on the resistance of the air outlet conduit. In this case, the relation between jet volume flow and air outlet flow dependent on the used air outlet conduit is to be adjusted by throttles within the air outlet channel and the blast air channel so that this method preferably is used for hoods operating in the recirculation mode. The suctioned airflow is divided into the blast jet and the recirculated jet. The recirculated air, similar to commercial hoods operating in the recirculation mode, can be blown into the area above the hood.
According to the second version described above, throttles can be dispensed with because a differentiation is made between the suction fan and the fan for generating the jet and the front which is called the vortex fan. A vortex fan blows out via the blow-off slot. The corresponding volume flow is dependent on the apparatus. A vortex fan is able to remove air by suction, as well as through its surface filter as by means of edge suctioning or from the surroundings above the hood, and a suction fan can be operated with both suction modes.
The suction flow field can be improved by means of corresponding structural designs. One possibility is to homogenize the steam. If the basic shape of the hood is a circular segment, an ellipsoid segment or a similarly curved shape, the front are at the wall connections of the exhaust hood only. A continuous ring-like shape without any lateral restrictions and irregularities of the front vortex is especially suitable for suspended exhaust hoods. This is true for all suctioning processes, which operate based on vortex flow or a front vortex for generating a front along the forward edge of the hood.
For cornered or rectangular exhaust hoods operating with edge suctioning, it is useful to partially interrupt the suction flow in order to obtain U-shaped vortex-type interruptions and to increase the length of the front. The width of said interruptions is approximately two-fold up to twenty-fold the thickness of the suction slot, whereas the length of the suction apertures is approximately two-fold up to thirty-fold the thickness of the suction slot. The length of the interruptions and apertures along the suction edge can be the same size or can be of different size.
With exhaust hoods or similar suction hoods without edge suctioning, the stream directed towards the filter surface is structured by tongue-like or wave-like formations of the suction surface. On locations where a tongue is positioned closer to the edge of the hood, an area of convergence is formed, whereas on those locations where a gap is provided between two adjacent tongues, an area of divergence is formed at the bottom side of the hood. A pair of longitudinal vortexes is associated to each tongue. The pair from adjacent gaps at the bottom side of the hood rotate towards the tongue and the exhaust effect.
If a blast flow is used for an exhaust hood generating a front, said flow can be formed by additionally corrugating the edge of the hood and the blow out slot. This is done in such a manner that the flow being diverted at the front side of the hood is provided with a component to the center line of the recess. The recesses or wave crests are areas of convergence, the wave troughs are areas of divergence below the hood. The results in longitudinal vortexes within the flow.
According to a special embodiment of an exhaust hood of the invention using the Coanda effect and a rectangular basic surface of the hood, it is useful to stagger the blow out aperture away from the front edge of the hood towards the interior (towards the center of the hood) in order to restrict the suction effect of the jet below the extension to the front half of the chamber underneath the exhaust hood. Compared with a blow out opening immediately at the front edge of the hood the suction effect of the jet is amplified in this manner. The distance is case of a special embodiment is approximately 50 mm. In general, it is sufficient to blow out at the front side of the hood only, whereby with a special embodiment the blow out slot is 4-5 mm, the blow out rate is 2-3 m/sec and the tube diameter is 38 mm. At the lateral restrictions of the tube circulated by air, longitudinal vortexes are formed which suppress the removal of the vapor at the lateral edges of the exhaust hood. In order to obtain a satisfying effect of these longitudinal vortexes, the vortexes also are to be arranged beyond a shield. The end of the blow out slot and the buve, therefore, is to be spaced from the lateral edges by about 50 mm. When operating in the recirculation mode, it is useful to let that part of the recirculating air which is not blown over the curvature, flow as slow as possible and over a large area. This exit location is spaced as far as possible from the front side of the hood because this flow is able to exert a suction effect onto the vapor so that the efficiency of the hood is considerably reduced.
According to a further special embodiment of the invention, the exhaust hood is formed so that two or more blasts jets are each provided with means for generating a curved shearing flow, which operate parallel to each other. A blast jet within the exhaust hood is divided into two separate jets, which overlap each other along their lateral area of curvature at the edge of the hood in such a manner that the outer curved wall is shorter than the inner curved wall. Two shearing flows spaced from each other will be obtained.
A special embodiment of the invention refers to a Coanda vortex hood. The blow out aperture is shifted or is spaced from the front edge of the hood towards the rear side. This restricts the suction effect of the jet below the extension to the semi-space, and amplifies the suction effect of the jet compared with a blow out aperture exactly at the front edge of the hood. In this case, it will be sufficient to blow out at the front side of the hood only. At the lateral restrictions of the tube circulated by air, longitudinal vortexes are formed, preventing the vapor from disappearing at the lateral edges of the hood. With comparable known systems, the longitudinal vortexes have been generated by special diversion means. For an acceptable structure of longitudinal vortexes, it is important that they are formed below a shield. The end of the blow out slot and the tube, therefore, is to be spaced distant from the lateral edges. In the recirculation mode, it is useful to make that part of the circulating air not blown out over the curvature exit as slow as possible and over a large area. The exit location is to be spaced as far as possible from the front edge of the hood because the flow can exert a suction effect onto the vapor or fumes which would reduce the efficiency of the hood.
A further embodiment of the invention refers to a combination of frontal vortexes with edge suctioning, whereby the effect of suctioning off will be improved. With frontal vortex hoods operating with edge suctioning effect, the experts differ between blast edges and suction edges of an exhaust hood. The blast edge is an edge for blowing off in order to generate a frontal flow directed toward the suction off apertures. A suction edge is an edge at which the air is removed by suction. The edges of an exhaust hood can be blast edges, blast and suction edges, suction edges or merely lateral edges (without any function as blast or suction edges).
The edge suction effect operates either with strip-like surface filters at the edge or with a slot at the edge, whereby the filter is arranged behind the slot.
Often, exhaust hoods do not justify the expenditure to arrange means for generating a frontal vortex along the entire hood edges or along the entire periphery of the hood. In these cases, a hood edge or part of the periphery of the hood will be provided with blow out apertures. The edge suction effect preferably is designed so that along a gap a very high suction speed substantially equivalent to the speed of the blast flow is generated. The channel is widened in order to keep the speed of air as low as possible when passing through the filter. However, a wall-type surface suction effect will be possible at the edges instead of a slot suction effect.
Improving the flow at the corners or at the end of the blow out means for generating a front vortex according to the invention is obtained by:
a) profiling the blow out means,
b) boundary layer suction,
c) positioning of the suction surface in a proper manner,
d) using an adhering jet, and
e) using a vortex tube.
The blow out flow of a front vortex hood at the lateral restrictions of the blow out slots is no longer xe2x80x9cquasi two-dimensionalxe2x80x9d. Experience shows that the flow at this location no long adheres as well on the lower edge of the hood, and sometimes is directed downwardly. In order to make the flow adhere as much as possible at the corners and to improve the stability, the surface of the curvature to be passed by air is profiled inclined less towards the end of the blow out aperture so that the tendency for the flow to separate caused by the shape is continuously decreased. This corresponds with the offset of a wing, the angle of adjustment of the profile of which decreases outwardly or the shape of the profile alters outwardly (geometrical and aerodynamical offset). In case of a freely profile body circulated by air, the offset can be made according to the profile of the wing.
With a further embodiment of the invention, a tube circulated by air is provided. The outer profile is a straight extension of a tangent to the curvature, whereas inside the blow out means the extension is increasingly shortened. The transient to the tube is designed as smooth as possible.
According to another embodiment lateral suction apertures close to the ends of the blow out means are provided for stabilizing the flow laterally. Furthermore, at the ends of the blow out means, a boundary layer suction effect can be provided.
Another alternative is to blow out a second wall jet, which in connection with a tube, acts as an adhering jet. This is adequate to a twin jet principle. The tube is provided with an inlet for the air of the adhering jet laterally within the interior of the hood. Below the hood a slot is arranged, from which the adhering jet exits. By positioning the inlet an d the outlet openings as well by diversion means the adhering jet can be directed inwardly.
Extending the frontal vortex or the curved shearing flow by means of an additionally general longitudinal vortex at the ends of the blow out means is a further alternative of the invention. Stabilizing the blast flow by off-setting, by boundary layer suction close to the suction surface or by an adhering jet, also can be used at other critical locations of the blow out device.
Furthermore, the invention proposes a suction device designed as a so-called vortex tube, where a radial and an axial flow are continuously merged so that this flow is formed into a rotating jet when exiting. This flow is suitable as an extension of a blow out flow. A vortex flow can be arranged at the outside of a tube around which the flow is passed. The tube also forms the air supply for the vortex tube. The air for the vortex tube also originates from the blast area of the hood and passes through the aperture within the tube through the inlet into the vortex tube. The jet flowing out from the exit aperture is diverted towards the suction surfaces. If the exit of the vortex tube is not centrically formed within the truncated cone, it will be below the bottom of the hood. However, the vortex tube also can be arranged sloping downwardly into the space below the bottom of the hood, and the truncated cone used for converging the flow can extend into the required direction. The rotational sense of the frontal vortex and of the longitudinal vortex is provided so that the longitudinal vortex forms an extension of the frontal vortex at the corners. A vortex tube is suitable for continuing the frontal flow structure laterally, if corner exhaust hoods are used. However, it can also be used to semi ring-shaped hoods, whereby the hollow body, such as a tube, can change into a vortex tube.
Further possibilities for making the flow at the lateral sides of the exhaust hood more stable are obtained by continuously decreasing the thickness of the blow out gap outwardly so that the relation of the thickness of the gap to the radius of curvature is decreased. From practical experiments with the Coanda effect, it si known that the angle of diversion of the flow is larger, the smaller the relation has been chosen. A further method is to increase the radius of the circular profile outwardly with constant thickness of the blow out slot. The profile or the way of blowing out air through the blow out device is to be designed so that there will be a longer contact time of the flow. This principle also is met by profiling the blow out device as mentioned above. Basically, the blow out device also is to be offset in a suitable manner, for example, by profiling or according to the geometrical offset of an airplane wing.